The present invention relates in general to illumination optics. It relates in particular to an illumination system wherein light is provided incident on a spatial light modulator.
A critical enabling technology for display systems in general, and most particularly laser based displays, is a spatial light modulator, or SLM. A spatial light modulator modulates intensity levels of light to impart information which is intended to be displayed on a display monitor. The information being content, which are light or dark levels for each pixel on the display.
One-dimensional spatial light modulators can be used for applications in high-resolution display systems. To impart a two-dimensional information field, systems using a one-dimensional spatial light modulator typically scan a column or row of light (depending on the scan direction) over the appropriate dimensions of the display field. Scanning can be accomplished by using a simple mirror mounted on a galvanometer. Many other conventional methods can also be used.
A general design consideration for any spatial light modulator based display is getting as much illumination onto the spatial light modulator as to provide the necessary illumination onto the display. As such, optimally directing light from multiple light sources onto the spatial light modulator is desirable. To maximize the illumination efficiency of the incident light on the spatial light modulator, constraints on the shape of the light beams and the angle to which the light beams illuminate the spatial light modulator need to be taken into account. Shaping and directing light beams under such tight constraints presents numerous challenges to the illumination optics.
A particularly effective device for use as a spatial light modulator is a reflective grating light valve type device array. Such displays are commonly known in the art. This type of reflective grating light valve type device array is capable of providing displays of very high resolution, very high switching speeds, and high bandwidth by virtue of the very small size (about 4xc3x97200 micrometers) of operable elements of the array. The very small operable elements can be operated electrostatically with low applied voltage. In combination with laser illumination and appropriate optics, a high resolution display can be achieved.
A significant problem in designing such a display system arises from the fact that the grating light valve type device array modulates light by diffraction, and light incident on the array for modulation is returned as a combination of reflected and diffracted light beams. Because of this, an optical system used with the display must be capable, not only of magnifying, focusing, or projecting an image of the grating light valve type device array to form a displayed image, but must also be capable of separating the diffracted light from the reflected light.
Separating light in such a manner puts an extremely tight constraint on the angular extent that the incident light beams on the grating light valve type device can have. The constraints on light beam shape and angle are even tighter for grating light valve type devices than those constraints imposed upon other SLM based display systems.
Also, conventional illumination of a spatial light modulator requires the use of multiple light sources. In the case of laser based displays, each light beam illuminates a small portion of the spatial light modulator which in turn illuminates a small portion on the display. Problems arise due to conventional implementation problems of keeping all the power disbursement of each light beam in balance. Problems also arise when a light source becomes inoperative. An inoperative light source will no longer illuminate its small portion of the spatial light modulator and consequently, a small portion on the display will not be illuminated. Typically, this is manifested as a small dark spot on the display. Such an occurrence is obviously highly undesirable.
What is needed is a means for multiple light sources to uniformly illuminate a spatial light modulator such that failing light sources do not result in missing images on the display. What is further needed is to provide such a uniform illumination within the tight optical constraints necessary to maximize the illumination efficiency of the multiple light sources.
The present invention is directed to illuminating a one-dimensional spatial light modulator using an illumination system employing multiple light sources. The illumination system comprises a parallel array of light sources which provides a plurality of light outputs to an optical train. The optical train effectively combines the light sources into a single light source. The single light source provides a single light output for uniformly illuminating the spatial light modulator.
The optical train includes a first optical train for receiving the light outputs from each light source, magnifying each light output, and overlaying each of the light outputs to form a single real magnified image. A mode conversion lens receives the single real magnified image, converts a mode profile of the single real magnified image into a top hat mode profile, and outputs a diverging light beam with a top hat mode profile. A second optical train shapes the light beam into an appropriate spatial geometry in such a manner that the light beam effectively illuminates the entire spatial light modulator, and directs the light beam onto the spatial light modulator.